Posey passed a concussion test, but his status for Thursday's series finale appears questionable.
"If he needs a day or two, he'll get it," Giants manager Bruce Bochy said.
That would mean either Joaquin Arias or Ehire Adrianza -- utility men who primarily have played infield -- would fill the role of backup catcher.
Games against the Marlins summon bad memories for the Giants. It was in this city in 2006 that catcher Mike Matheny absorbed such a shock from a foul tip that he was forced to retire. But Posey accepted the punishment as one of the consequences of playing behind the plate.
"I've had these before. I've been lucky this year," said Posey, 28. "I really haven't had any that have zinged me. This was the first one that caught me directly on the chin."
Posey has started 53 games at catcher and 17 games at first base. He's batting .295 when he starts behind the plate and .333 as a first baseman.
Chris Haft is a reporter for MLB.com. Read his blog, Haft-Baked Ideas, follow him on Twitter at @sfgiantsbeat and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.